1. Technical Field
The present invention relates to a tunable dispersion compensator using a plurality of Mach-Zehnder interferometers cascaded on a planar lightwave circuit.
2. Related Arts
So far, there are following two technologies for example as methods for dissolving a polarization dependency regarding a double-pass planar lightwave circuit type tunable dispersion compensator (TDC) using a plurality of Mach-Zehnder interferometers (MZIs) cascaded on a planar lightwave circuit (PLC).
1. In a published Japanese patent application No. 2005-092217 (hereinafter, it is described as a document 1), there is disclosed a technology for dissolving the polarization dependency by arranging a half-wave plate with crossing each middle point of two arms of a middle positioned MZI of cascaded three MZIs.
2. In a document by C. R. Doerr et. al, IEEE Photonics Technology Letters, Vol. 17, No. 12, December 2005, pp. 2637-2639 (hereinafter, it is described as a document 2), there is disclosed a technology for dissolving the polarization dependency by arranging a reflection mirror on one end facet of a PLC chip in which four MZIs are formed, and by inserting a quarter-wave plate between the one end facet and the reflection mirror.
According to the conventional technology of the above mentioned document 1, however, there are problems that a total manufacturing cost becomes increased because a loss on a reflection surface of the mirror is increased and a high accuracy of mounting the mirror is required, when a double-pass is obtained with being reflected by the mirror arranged on the one end facet of the PLC chip for enhancing an amount of dispersion.
On the contrary, according to the conventional technology of the above mentioned document 2, it is possible to rotate a polarization by using the quarter-wave plate at the same time of reflecting by the reflection mirror, however, there are problems that a loss on the reflection surface of the reflection mirror is large, and the loss on the reflection surface becomes increased due to a small displacement of an angle on the one end facet of the PLC chip.